1. Field of the Invention
The present invention relates to a motor control apparatus for controlling a motor and a control method thereof, and particularly to a motor control apparatus for controlling a motor having a sensor and a control method thereof.
2. Description of Related Art
In recent years, there are increasing demands for linear rotation of a motor, attracting attention to techniques to accurately control a motor. One of the techniques is a technique to use a sensor to detect a rotation angle and speed, particularly a technique using a resolver. The resolver is a kind of rotary transformer that includes a stator and a rotor. The resolver detects a position of the rotor rotating correspondingly to a motor rotation by an output from the stator. The resolver is generally supplied with an excitation power supply. Based on the excitation power supply, a signal corresponding to the position of the stator and the rotor is outputted from the resolver. The stator is usually placed to 90° rotated position. Thus the resolver outputs two kinds of output signal modulated by sine and cosine waves. This is disclosed in “Sougou Denshi Publishing Company, Basics and Applications of Brushless Servo Motor, pp. 72-74, June, 1985.
In the conventional motor control method using a resolver, sine and cosine wave output signals, which are analog signals, are digitalized using a R/D converter (resolver/digital converter). This technique is disclosed in Japanese Unexamined Patent Application Publication No. 2004-301806. The excitation power supply to the resolver is generated by converting sine wave digital data stored to data ROM (Read Only Memory) into analog data by DAC (Digital-to-Analog Converter), filtering (integrating) the analog data by LPF (Low Pass filter) and amplifying the analog data.
The technique disclosed in Japanese Unexamined Patent Application Publication No. 2004-301806 compares the digital signal outputted from the R/D converter with an expected value to detect a rotation position. Based on the detected rotation position of a motor, a driving current to drive the motor is controlled. This allows a feedback control on the rotation speed of a motor. Specifically, in case the current rotation speed of the motor is detected to be faster than a desired speed by an output signal from the resolver, the driving current is controlled to slow the rotation speed. On the other hand in case the rotation speed is slower than the desired speed, the driving current is controlled to speed up the rotation speed.
At this time, the timing to sample the output signal from the resolver is previously determined. For example samplings are performed for 3 timings at 0, 90, and 180 degrees. For a sine wave, at 0 and 180 degrees, it indicates a center value of an amplitude of a sine wave for excitation. At 90 degrees, it indicates the maximum amplitude value of a sine wave for excitation. Therefore, by comparing a center value of an amplitude of a sine wave for excitation, an expected maximum amplitude value, and an actual measurement value that sampled from a sine and a cosine waves outputted from the resolver, it is possible to detect a difference. Accordingly controlling a driving current gives a feedback.
However amplitudes of sine and cosine waves vary due to a heat generated by a motor, a heat of an apparatus controlled by the motor, a fluctuation in lines from a resolver to the motor control apparatus, a fluctuation in amplifier circuit for amplifying sine and cosine waves, and a fluctuation in power supply voltage supplied to the amplifier circuit, because the resolver is mounted near the motor. For example in case an amplitude of a sine wave from the resolver becomes smaller, the small amplitude is the maximum value, thereby making a dynamic range smaller. Specifically, as a signal outputted from the resolver is an analog signal, an A/D (Analog-to-Digital) conversion is performed to process the signal inside the MCU (MicroContoller Unit). A resolving power of an A/D conversion circuit is configured assuming that an amplitude is an expected value. Accordingly if the amplitude only reaches 50% of the expected value, the resolving power of the A/D conversion circuit relatively becomes smaller. This causes an accuracy of motor control to be decreased.
Further, in case a motor angle is detected by comparing an amplitude of an output signal from the resolver with an expected value, an amplitude value itself may change, generating another problem of a detection of an incorrect motor angle.
The present invention is made in light of the above issues and is purported to provide a motor control apparatus a motor control circuit and a method thereof capable of a control with high accuracy.